The hydraulic system used in such machines is provided with a main control valve which controls the flow of hydraulic oil to a plurality of hydraulic piston-cylinder motors which operate associated parts of the machine in question.
The main control valve comprises a number of slides which are activated by a servo-valve which incorporates a converter. In principle, the converters comprise a plunger which is axially movable in a cylindrical barrel or bore. The piston is activated electromechanically, and is displaced so that oil, i.e. pilot oil, is able to bypass the converter and therewith act upon the valve slides.
The present invention relates to the supply of oil to such servo-assemblies.
One known method of supplying oil to a servo-valve is to provide a separate oil pump therefor. This arrangement is encumbered with several disadvantages, however, and consequently the most common method used is one of providing a pump for the whole system and to channel off a given flow of oil from the pump and deliver it to the servo-valve. The invention relates to this latter type of system, which lacks the provision of a separate servo-pump.
In one such system, which is well known to the art, there is incorporated downstream of the pump a pressure compensated flow distributor, a so-called priority valve, which takes a part of the oil from the pump and delivers it to the servo-valve, while the remainder of the oil is conducted to the main valve. Arranged between the flow distributor and the servo-valve is a pressure control valve which opens at a given pressure, whereupon the divided oil flow is shunted directly to a tank. The flow distributor sends a constant flow of oil to the servo-valve.
The main disadvantage with this known system is that the set amount of oil passed to the servo-valve must always be greater than the maximum amount of oil consumed thereby.
Consequently, when the servo-valve is not activated, a considerable amount of oil will drain-off to the tank, via the pressure control valve, which results in unnecessary losses. In addition, the flow distributor must work at an internal pressure drop, in order to have a smooth and well-defined characteristic. If, on the other hand, the flow distributor is designed to work at the lowest possible drop in pressure, in order to avoid pressure losses, malfunctioning is likely to occur in the machinery being served. If the internal pressure drop is high, on the other hand, the pressure under which the main flow stands will naturally be affected, resulting in significant losses.
In another known, but less common system the flow distributor is replaced with a back pressure valve located between the pump and the main valve. This back-pressure valve is set for a pressure such that the valve will not open until a pressure which exceeds the necessary servo pressure is reached. A typical back pressure in this respect is 25 bars. Consequently, the resultant pressure losses are quite considerable, since the pump must constantly produce a pressure which is 25 bars higher than the pressure required at the moment in the main valve.
Both of these servo-valve oil supply systems result in significant losses when in operation.
Consequently, there is a need to design such a system in which the losses are considerably lower, or nonexistent.
This need is filled by means of the present invention.
According to the invention, there is provided a system in which losses are substantially eliminated.